Method for carbon dioxide snow separation

ABSTRACT

Improved method and apparatus for separating carbon dioxide snow from a mixture of carbon dioxide snow and vapor, without reference to gravity, in order to deliver carbon dioxide snow and vapor each at a substantial rate of flow.

United States Patent 1191 Campbell 1 Sept. 11, 1973 [54] METHOD FORCARBON DIOXIDE SNOW 3,528,221 9/1970 Garrett 55/461 SEPARATION 1,382,8466/1921 Merrill 1,546,682 7/1925 Slate Inventor: Ronald R Campbell, OakLawn, 1,870,691 8/1932 Rust 62/10 I 73] Assignee: Chemetron Corporation,Chicago,

1"- Primary ExaminerNorman Yudkoff Feb. 8 Assistant Examiner-A. PurcellAtt0rneyNicholas M. Esser I21] Appl. No.1 113,489

52 us. c1 62/10, 62/12, 55/461 1571 ABSTRACT [51 Int. Cl. F25 l/00, F253/00 Improved method and apparatus for Separating carbon [58] Held ofSearch ..62/9,10,l1, 12,

55/46] d10x1de snow from a mlxture of carbon d10x1de snow and vapor,without reference to gravity, in order to. de- I 561 References Cited l1ver carbon d10x1de snow and vapor each at a substantlal rate of flow.UNITED STATES PATENTS 2,929,] 12 11/1960 Massey 55/461 16 Claims, 5Drawing Figures PATENTEDSEPI Hm 3.157. as?

Inventor Ronald F. Campbell H'bborneg/ BACKGROUND OF THE INVENTION Thisinvention pertains to an improved method and improved apparatus forseparating carbon dioxide snow from a mixture of carbon dioxide snow andvapor.

When liquefied carbon dioxide flows through a restriction in a conduitsuch that the pressure of the liquefied carbon dioxide is reduced from apressure above the triple point pressure (approximately 75 psia) to apressure below the triple point pressure, the liquefied carbon dioxideis converted to a mixture of snow and vapor. In many practicalapplications, as where carbon dioxide snow is to be employed as arefrigerating medium and carbon dioxide vapor is to be recovered andremoved from the presence of operating personnel or recycled, it isnecessary to separate carbon dioxide snow from the mixture.

By way of background, known apparatus of the cyclone type for separatingparticulate matter from gaseous matter has been used for separatingcarbon dioxide snow from a mixture of carbon dioxide snow and vapor. Forexample, see U.S. Pat. No. 1,546,682. In such apparatus, in which themixture is injected generally tangentially through-an opening in thelateral wall of an upright chamber, carbon dioxide snow is separatedfrom the mixture through the combined actions of centrifugal force andgravity and thus settles toward the bottom of the chamber, andcarbondioxide vapor is drawn off through an opening in the top of the chamber.Thus, such apparatus relies upon gravity for separation of carbondioxide from such a mixture.

According to well known principles, once any object or material has beenset in motion, the force requiredto deflect the object or material frommotion in a straight line depends upon the velocity and the density ofthe object or material being deflected. In a flowing mixture of carbondioxide snow and vapor, in which the snow is many times denser than thevapor and in which the velocities of the snow and the vapor are similar,the snow offers more resistance to a deflecting force. Thus, where thedeflecting force is provided by a curved surface along which the mixtureflows, the snow tends to move under centrifugal force to the outer sideof the curvilinear flow path bounded by the curved surface and todisplace th'e'vapor away from the curved surface. In' the prior art,these principles have been applied to separation of carbon dioxide snowfrom a mixture of carbon dioxide snow and vapor for controlled dischargeof carbon dioxide snow andvapor from fire ex tinguishing nozzles andsimilar a'pparatusfor example, see U.S. Pat. Nos. 2,322,758,: 2,357,039,and 2,387,935. In such apparatus, vapor and snow are dischargedtogether, and no provision is made for recovery of vapor. v

Herein, the efficiency of recovery of carbon dioxide snow from liquefiedcarbon dioxide flashed to a mixture of carbon dioxide snow and vapor isdefined as the percent by weight of the .total liquefied carbon dioxideflashed which is recovered as solid. For example, where carbon dioxidesnow is recovered at a rate of 30 pounds' per minute from liquefiedcarbon dioxide flashed at a rate of 100 pounds per minute, theefficiency is 30 percent. In such an example, the theoretical efflciencydepends upon the temperature and pressure of liquefied carbon dioxideflashed and by way of example would be approximately 48 percent forsaturated liquefied carbon dioxide under a source pressure of 238 psig.

It has been observed that particles of carbon dioxide snow may assumeeither a flaky form or a granular form. Particles of carbon dioxide snowin a flaky form, which have more surface area and thus tend to meet moreresistance to movement through the atmosphere, may be better suited tosome practical applications, as where carbon dioxide snow is to bepacked gently around fragile items such as foodstuffs. Particles ofcarbon dioxide snow in a granular form, which have less surface area andthus tend to meet less resistance to movement through the atmosphere,may be better suited to other practical applications, as where carbondioxide snow is to be projected over a substantial distance. It has beenfound that the method and apparatus of this invention may be appliedboth to practical applications where particles. of carbon dioxide snowin a flaky form are useful and to practical applications where particlesof carbon dioxide snow in a granular form are useful.

Reference may be made to U.S. Pat. Nos. 3,320,075 and 3,498,799. Thesepatents exemplify uses of carbon dioxide snow as a refrigerating mediumand suggest typical settings for applications of the method andapparatus of this invention.

Reference also may be made to U.S. Pat. No. 2,988,898. This patent,which generally pertains to conversion of liquefied carbon dioxide to amixture of carbon dioxide snow and vapor, discloses a process useful inapplications of the method and apparatus of this invention whereby thepressure in a tubular conduit just downstream of an orifice throughwhich liquefied carbon dioxide is flashed should be maintained between 5and 60 psig for flow of the resultant mixture-of carbon dioxide snow andvapor through the tubular conduit.

From the foregoing, it is concluded that there remains a need for amethod and apparatus for efficiently separating carbon dioxide snow froma mixture of carbon dioxide snow and vapor, without reference togravity, for delivery of carbon dioxide snow at a substantial rate offlow with provision for recovery of vapor at a substantial rate of flow.

SUMMARY OF THE INVENTION Broadly, this invention contemplates flowing amixture of carbon dioxide snow and vapor along a curvilinear surfacesuch that carbon dioxide snow is concentrated along the curvilinearsurface by application of centrifugal force and dividing concentratedcarbon dioxide snow from the remainder ofthe mixture for withdrawalacross a margin of the curvilinear surface. Accordingly, it has beenfound that highly efficient separation of carbon dioxide snow from amixture of carbon dioxide snow, and vapor may be attained.

Thus, this invention provides a highly efficient method and apparatusfor separating carbon dioxide snow from a mixture of carbon dioxide snowand vapor, without reference to gravity, for delivery of snow at asubstantial rate of flow with provision for recovery of vapor at asubstantial rate of flow.

In accordance with the principles of this invention, these objects maybe attained in apparatus comprising a tubular conduit having an inletend and an outlet end, means for introducing a mixture of carbon dioxidesnow and vapor into the conduit for flow from the inlet end toward theoutlet end, at least a portion of the conduit between the inlet end andthe outlet end being curved to define a curvilinear flow path for themixture such that carbon dioxide snow tends to be concentrated for flowin a concentrated stream through the curvilinear flow path, anddeflecting means for deflecting a portion of the mixture for flowthrough an orifice in the lateral wall of the conduit. Preferably, thedeflecting means comprises a blade positioned to deflect theconcentrated stream of carbon dioxide snow through the orifice, and theorifice is located in the lateral wall of the curved portion of theconduit and opens radially outwardly. Additional features of theapparatus may comprise means for applying back pressure to the conduitand means for defrosting the blade. Also, the apparatus may be combinedwith flashing means for flashing liquefied carbon dioxide to introduce amix-.

ture of carbon dioxide snow and vapor into the conduit for flow from theinlet end toward the outlet end.

Similarly, these objects may be attained in a method comprising thesteps of flowing a mixture of carbon dioxide snow and vapor through atubular conduit defining a curvilinear flow path such that carbondioxide snow tends to be concentrated for flow in a concentrated streamand dividing the concentrated stream of carbon dioxide snow from theremainder of the mixture. Preferably, the concentrated stream of carbondioxide snow is deflected for flow radially outwardly along a flow pathintersecting the curvilinear flow path. The method may comprise theadditional step of applying back pressure to the curvilinear flow path.Also, the method may comprise the additional step. of flashing liquefiedcarbon dioxide to introduce a mixture of carbon dioxide snow and vaporinto the curvilinear flow path.

Other objects of this invention are to increase the efficiency ofrecovery of carbon dioxide snow by controlled application of backpressure and to control production of granular particles of carbondioxide snow by control of the rate at which liquefied carbon dioxide isflashed.

These and other objects, features and advantages of this inventionshould be evident from the following description, with the aid of theaccompanying drawing, of a preferred embodiment of this invention.

I BRIEF DESCRIPTION OF THE DRAWING In the drawing:

FIG. 1 is an elevational view, partly. in cross-section, showingapparatus embodying this invention as included in a system fordelivering carbon dioxide snow for use as a refrigerating medium;

FIG. 2 is an elevational view taken substantially along line 2--2 ofFIG.l; 4

FIG. 3 is a fragmentary view, on an enlarged scale, showing a detail ofthe apparatus of FIG. 1;

FIG. 4 is a sectional view, taken substantially along line 4-4 of FIG.1; and

FIG. 5 is a fragmentary view, somewhat similar to FIG. 3, showing adetail of a modification of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, apparatus embodyingthis invention for separating carbon dioxide snow from a mixture ofcarbon dioxide snow and vapor is shown as included in a system S fordelivering carbon dioxide snow for uses as a refrigerating medium. Asshown, a box B containing perishable foodstuffs or the like is supportedby means of a conveyor C for movement beneath a hood H shielding theapparatus 10. Uses of carbon dioxide snow as a refrigerating medium arewell known and are exemplified in the aforementioned U.S. Pat. Nos.3,320,075 and 3,498,799.

The system S also includes a source 12 of liquefied carbon dioxide, aconventional ball valve 14 connected and arranged to control the flow ofliquefied carbon dioxide from the source 12 to the apparatus 10 througha tubular conduit 16 leading from the source 12 to the valve 14 andthrough a tubular conduit 18 leading from the valve 14 to the apparatus10, and an orifice nozzle 20 connected between the tubular conduit 18and the apparatus 10. As shown, the inlet end 22 of the orifice nozzle20 is externally threaded for threaded connection to an internallythreaded end portion 24 of the tubular conduit 18, and the outlet end 26of the orifice nozzle 20 similarly is externally threaded for threadedconnection to an internally threaded coupling 28.

As shown, the outlet end 26 of the orifice nozzle 20 includes an orificeplate 30, which has an orifice 32 drilled or otherwise formed therein todefine a restricted passageway for liquefied carbon dioxide flowing fromthe source 12 to the apparatus 10 through the orifice nozzle 20.Accordingly, as is well known, the pressure of the liquefied carbondioxide may be reduced from a pressure above the triple point pressure(approximately psia) to a pressure below the triple point pressure suchthat the liquefied carbon dioxide is converted to a mixture of carbondioxide snow and vapor.

The apparatus 10, which is supported generally within the hood H of thesystem S, generally comprises a tubular conduit 40. As shown, thetubular conduit 40 has an inlet end 42, which is adapted to receive amixture of carbon dioxide snow and vapor for flow through the tubularconduit 40, and an elongated straight inlet end portion 44, whichincludes the inlet end 42. As shown, the coupling 28 is is brazed orotherwise suitably mounted to the inlet end 42 of the tubular conduit40. The tubular conduit 40-further has an outlet end 46, which isadapted to discharge a portion of the mixture flowing through thetubular conduit 40, an elongated straight outlet end portion 48, whichincludes the outlet end 46, and an elongated curved portion 50 betweenthe inlet end portion 44 and the outlet end portion 48. The apparatus 10further generally comprises means 52 for deflecting a portion of themixture for flow through an orifice 54 in the lateral wall 56 of thecurved portion 50 of the tubular conduit 40.

By application of the process disclosed in the aforementioned U.S. Pat.No. 3,988,898, the size of the orifice 32 should be regulated such thatthe pressure within the inlet end portion 44 of the tubular conduit 40is maintained between 5 and 60 psig. Thus, the carbon dioxide vapor inthe tubular conduit 40 will flow .at a velocity sufficient to keep theaccompanying carbon dioxide snow in suspension for flow through thetubular conduit 40.

In order to minimize the length of the curved portion 50 of the tubularconduit 40 necessary for'efficient concentration of carbon dioxide snow,the inlet end portion 44 of the tubular conduit 40 should be elongatedto permit the mixture to fill the cross-section of the tubular conduit40 before entering the curved portion 50 of the tubular conduit 40. Asshown, the length of the inlet end portion 44 of the tubular conduit 40is approximately one-fourth of the overall length of the tubular conduit40.

According to the previously mentioned well known principles, once anyobject or material has been set in motion, the force required to deflectthe object or material from motion in a straight line depends upon thevelocity and the density of the object or material being deflected, and,where the deflecting force is provided by a curved surface along whichthe mixture flows, the

nally threaded for threaded connection to the coupling 76. The outletend 82 of the reducing bushing 78 is of reduced internal diameterrelative to the inlet end 80 and is internally threaded for threadedconnection to denser snow tends to move along the curvilinear flow pathbounded by the curved surface and to displace the less dense vapor awayfrom the curved surface. In the apparatus 10, the curved portion 50 ofthe tubular conduit 40 defines a curvilinear flow path of generallyuniform tubular cross-section for the flowing mixture of carbon dioxidesnow and vapor. Thus, by application of these principles, the snow tendsto move under centrifugal force to the outer side of the curvilinearflow path and to displace the vapor away from an outer portion 58 of thelateral 'wall 56 of the curved portion 50 of the tubular conduit 40.Accordingly, the snow tends to be concentrated for flow in aconcentrated stream in the curved portion 56 of the tubular conduit 40.In order to promote concentration of the snow, the curved portion 50 ofthe tubular conduit 40 should define an areaate flow path extending overapproximately from 90 to 180 of arc.

As shown, the orifice 54, which opens radially outwardly, is located inthe curved portion 50 of the tubular conduit'40, and the deflectingmeans 52, which comprises a blade 60 brazed or otherwise suitablymounted to the lateral wall 56 of the tubular conduit 40 at a downstreammargin 62 of the opening 54, is-positioned to divide the concentratedstream of carbon dioxide snow from the remainder of the mixture and todeflect the concentrated stream of carbon dioxide snow through theopening 54 along a flow path intersecting the curvilinear path definedby the curved portion 50 of the tubular conduit 40. For convenience inmounting the blade 60, asshown, the tubular conduit 40 may be made intwo sections, 40a and 40b, respectively, brazed or otherwise suitablyjoined together at a connection between a flange64a on the section 40aand a flange 64b on the section 40b. As shown, the blade 60 is generallyin the form of one half-section of'a'tube, and the leading edge 66 ofthe blade 60 is knife-edged.

Since there is a tendency for water vapor from the atmosphere-to formfrost on the blade 60, particularly when the apparatus has been shutdown after use, it is useful for the apparatus further to comprise means70 for defrosting the blade 60. As shown in W6. 1, the defrosting means70 may be in the form of an electrical resistance heater 72 externallymounted to the blade 60.

As explained hereinafter, it is useful for the pressure within thetubular conduit 40 to be maintained above atmospheric pressure,Accordingly, the apparatus 10 further comprises means 74 for applyingback pressure to the tubular conduit 40. As shown, an externallythreaded coupling 76 is brazed or otherwise suitably mounted to theoutlet end 46 of the tubular conduit 40,

the threaded end 84 of a tubular conduit 86. The tubular conduit 86permits carbon dioxide vapor to be drawn off and recycled or otherwisedirected to a point of use. Where carbon dioxide vapor merely needs tobe vented, the tubular conduit 86 may be omitted.

Controlled application of back pressure to the tubular conduit 40.maintains the pressure within the tubular conduit 40 above atmosphericpressure and has been found to significantly increase the efficiency ofrecovery of carbon dioxide snow. Where it is believed that an efficiencyof 33 percent ordinarily would be considered favorable, efficienciesgreater than 38 percent may be obtained with the apparatus 10.

In order to take full advantage of the length of the curved portion 50,the blade 60 should be positioned near the junction of the curvedportion 50 and the outlet end portion 48. However, in order to avoid anysubstantial recombination of the concentrated stream of snow with theremainder of the mixture, the, blade 60 should not be positionedimmediately at the junction of the curved portion 50 and the outlet endportion 48. It has been observed that the concentrated stream of carbondioxide snow assumes theform of thin narrow ribbon while flowing throughthe curved portion 50 of the tubular conduit 40. Thus, in order tominimize the amount of carbon dioxide vapor discharged through theorifice 54, the blade 60 may be positioned with its leading edge 66extended only a slight distance into the tubular conduit 40, as shown inFIGS. 1 and 3.

The apparatus 10 efficiently separates carbon dioxide snow from amixture of carbon dioxide snow and vapor without reference to gravity.Of course, as in the system S, gravity may be used to assist thedelivery of separated carbon dioxide snow to a point of use. Theapparatus 10, which delivers carbon dioxide snow, through the orifice 54at a substantial rate of flow,-also provides for recovery of carbondioxide vapor through the outlet end 46 of the tubular conduit 40 at asubstantial rate of flow. Thus, the recovered carbon dioxide tubing, 24.inches longgBoththe'straightinlet end p'ortion of the tubular conduitand the straight outlet'end portion of the tubular conduit were 6.125inches long.

The curved portion of the tubular conduit defined an arcuate flow pathfor the mixture extending over degrees of arc. The radius of curvatureof the curved portion of the tubular conduit, when measured to thecentral axis of the tubular conduit, was 3.75 inches long. The blade wasformed from a half-section of similar tubing and was positioned'with itsleading edge at 21 degrees of are from the junction of the curved andoutlet end portions ofthe tubular conduit and thus at 159 of are fromthe junction of the curved and inlet end portions of the tubularconduit. Reducing bushings of varying internal diameters were used toapply varying back pressures to the tubular conduit. The apparatus wasoperated at ambient temperatures.

In a first series of tests utilizing the apparatus described in thepreceding paragraph, the efficiency in terms of the weightof carbondioxide snow recovered as divided by the weight of liquefied carbondioxide flashed was noted for various back pressures applied to thetubular conduit. The flowing results were noted:

NOTE: For saturated liquefied carbon dioxide under a pressure of 238psig at the source, the theoretical efficiency would be approximately 48percent. It is believed that an efficiency of 33 percent ordinarilywould be considered favorable by those skilled in the art.

From these results, it has been noted that a back pressure of 20 psig orless increased the efficiency of recovery of carbon dioxide snow, andthat back pressures of 10 or less psig provided efficiencies greaterthan the efficiency provided by a back pressure of 20 psig. Accordingly,it has been found that the efficiency may be increased by controlledapplication of back pressure to maintain the pressure within the tubularconduit above atmospheric pressure.

In a second series of tests of utilizing the same apparatus, theappearance of granular particles of carbon dioxide snow among flakyparticles of carbon dioxide snow was related to the rate of discharge ofcarbon-dioxide snow (as determined by the rate at which liquefied carbondioxide was flashed and thus determined by the inlet pressure ofliquefied carbon dioxide and by the size of the inlet orifice throughwhich liquefied carbon dioxide was flashed). The following results werenoted:

' SNOW LOSS AND RESTRICTION Inlet Snow Snow Orifice Discharge Velocity-Test P, P, P, Size Rate Appearance I 297 290 4 0.157" 9% Good Flakelb./min. (Low Velocity) 2 294 289 4% 0.l6l" l2 Excellent lb./min. Flake(Low Velocity) 3 292 287 4% 0.l695" 14% Flake lb./min.

4 291 283 4% 0.177" 15 Flake (K) lb./min.

5 290 286 5% 0.182" Good Flake lb./min.

6 290 286 5% 0.189" I65 Flake (0K) lb./min.

7 288 285 5% 0.l935" l8 Flake & GranulbJmin. lar (Velocity P, Tank(Source) Pressure (psig) P, Pressure at Inlet Orifice (psig) P. BackPressure (psig) v 7 NOTE: Granular particles appeared at snow dischargerates greater than approximately 16 lb./min.

From these results it has been noted that the appearance ordisappearance of granular particles of carbon dioxide snow among flakyparticles of carbon dioxide snow depended upon the rate at whichliquefied carbon dioxide was flashed. Accordingly, it has been foundthat production of granular particles of carbon dioxide snow may becontrolled by control of the rate at which liquefied carbon dioxide isflashed.

In the modification of FIG. 5, in which primed reference numbers areused to indicate parts similar to likenumbered parts in FIGS. l-4 exceptas mentioned below, an enlarged orifice 54 opens radially inwardly andis located in the curved portion 50' of a tubular conduit A chamber 100,which has an outlet 102, is mounted to the tubular conduit 40' toreceive carbon dioxide vapor from the orifice 54'. A tubular conduit 104is connected to the chamber 100 for communication with the outlet 102.The blade 60 is elongated in cross-section, as shown, such that itsleading edge 66 is extended a substantial distance into the tubularconduit 40 Accordingly, while the concentrated stream of carbon dioxidesnow and a minimum amount of carbon dioxide vapor are permitted to flowto the outlet end (not shown) of the tubular conduit 40, carbon dioxidevapor may be drawn off through the tubular conduit 104.

Where desired, a second apparatus similar to the apparatus 10 may beseries-connected to the apparatus 10 with the inlet end of the secondapparatus connected to the outlet end 46 of the apparatus 10. Also,where desired, carbon dioxide vapor recovered from the outlet end 46 ofthe apparatus 10 may be conducted in heat exchange relationship eitherto the conduits 16 and 18 or to the tubular conduit 40, or to both,whereby heat input from the ambient atmosphere may be reduced.Furthermore, the apparatus 10 may be operated for recovery of carbondioxide vapor through the outlet end 46 at high pressures approachingthe triple point pressure.

The tubular conduit 40 confines the flowing mixture of carbon dioxidesnow and vapor and thus markedly reduces the sound level in the ambientatmosphere from the ordinarily objectionable sound level that would bereached if liquefied carbon dioxide were to be flashed into the ambientatmosphere. Similarly, withdrawal of carbon dioxide vapor from theoutlet end 46, as through the tubular condiut 86 further reduces thesound level in the ambient atmosphere.

Thus, it willbe appreciated that all of the recited objects, advantagesand features of this invention have been demonstrated as obtainable in ahighlypractical apparatus and one that is simple and positive inoperation. It will be further understood that although this inventionhas been described with respect to certain specific embodiments thereof,this invention is not limited thereto, since various modifications ofsaid invention will suggest themselves from the aforesaid descriptionand are intended to be encompassed within the scope of the appendedclaims.

I claim:

1. Method for separating carbon dioxide snow from a mixture of carbondioxide snow and vapor comprising the steps of expanding liquid carbondioxide to a pressure below the triple point to form carbon dioxide snowand vapor introducing a flowing mixture of carbon dioxide snow and vaporthrough one end of a tubular conduit curved to define a curvilinear flowpath axially through the tubular conduit such that carbon dioxide snowtends to be concentrated in a stream flowing axially through the tubularconduit, deflecting the concentrated stream of carbon dioxide snow fromthe remainder of the mixture, and discharging the concentrated stream ofcarbon dioxide snow from the tubular conduit.

2. The method of claim 1 wherein the curvilinear flow path is arcuate.

3. The method of claim 2 wherein the curvilinear flow path extends overapproximately from 90 to 180 of are.

4. The method of claim 1 further comprising the step of flashingliquefied carbon dioxide to introduce the mixture of carbon dioxide snowand vapor into said end of the tubular conduit for flow of the mixtureaxially through the tubular conduit along the curvilinear flow path.

5. The method of claim 1 wherein liquefied carbon dioxide is flashedthrough a restricted flow path com municating with the curvilinear flowpath.

6. The method of claim 1 wherein the steps-are performed at a pressureabove atmospheric pressure.

7. The method of claim 1 further comprising the step of applying backpressure to the curvilinear flow path.

8. The method of claim 6 wherein the steps are performed at a pressurewhich does not exceed 20 psig.

9. The method of claim 6 wherein the steps are performed at a pressurewhich does not exceed 10 psig.

10. The method of claim 6 wherein the steps are performed at a pressurewhich does not exceed 7 psig.

11. The method of claim 7 wherein the remainder of the mixture flowsthrough a restricted flow path communicating with the curvilinear flowpath.

12. The method of claim 1 wherein the concentrated stream of carbondioxide snow is deflected for flow along a flow path intersecting thecurvilinear flow path.

13. The method of claim 12 wherein the concentrated stream of carbondioxide snow is deflected for flow radially outwardly through an openingin the lateral wall of the tubular conduit.

14. The method of claim 12 further comprising the step of flashingliquefied carbon dioxide at a controlled rate to introduce a mixture ofcarbon dioxide snow and vapor into the curvilinear flow path and tocontrol production of granular particles of carbon dioxide snow for flowin the flow path intersecting the curvilinear flow path.

15. The method of claim 14 further comprising the step of reducing therate at which liquefied carbon dioxide is flashed to retard productionof granular particles of carbon dioxide snow for How in the flow pathintersecting the curvilinear flow path.

16. The method of claim 14 further comprising the step of increasing therate at which liquefied carbon dioxide is flashed to promote productionof granular particles of carbon dioxide snow for flow in the flow pathintersecting the curvilinear flow path.

2. The method of claim 1 wherein the curvilinear flow path is arcuate.3. The method of claim 2 wherein the curvilinear flow path extends overapproximately from 90* to 180* of arc.
 4. The method of claim 1 furthercomprising the step of flashing liquefied carbon dioxide to introducethe mixture of carbon dioxide snow and vapor into said end of thetubular conduit for flow of the mixture axially through the tubularconduit along the curvilinear flow path.
 5. The method of claim 1wherein liquefied carbon dioxide is flashed through a restricted flowpath communicating with the curvilinear flow path.
 6. The method ofclaim 1 wherein the steps are performed at a pressure above atmosphericpressure.
 7. The method of claim 1 further comprising the step ofapplying back pressure to the curvilinear flow path.
 8. The method ofclaim 6 wherein the steps are performed at a pressure which does notexceed 20 psig.
 9. The method of claim 6 wherein the steps are performedat a pressure which does not exceed 10 psig.
 10. The method of claim 6wherein the steps are performed at a pressure which does not exceed 7psig.
 11. The method of claim 7 wherein the remainder of the mixtureflows through a restricted flow path communicating with the curvilinearflow path.
 12. The method of claim 1 wherein the concentrated stream ofcarbon dioxide snow is deflected for flow along a flow path intersectingthe curvilinear flow path.
 13. The method of claim 12 wherein theconcentrated stream of carbon dioxide snow is deflected for flowradially outwardly through an opening in the lateral wall of the tubularconduit.
 14. The method of claim 12 further comprising the step offlashing liquefied carbon dioxide at a controlled rate to introduce amixture of carbon dioxide snow and vapor into the curvilinear flow pathand to control production of granular particles of carbon dioxide snowfor flow in the flow path intersecting the curvilinear flow path. 15.The method of claim 14 further comprising the step of reducing the rateat which liquefied carbon dioxide is flashed to retard production ofgranular particles of carbon dioxide snow for flow in the flow pathintersecting the curvilinear flow path.
 16. The method of claim 14further comprising the step of increasing the rate at which liquefiedcarbon dioxide is flashed to promote production of granular particles ofcarbon dioxide snow for flow in the flow path intersecting thecurvilinear flow path.